The present invention concerns a journal through which flows a heat transfer medium and which is designed as a hollow shaft for a hollow roll body, specifically for a steam-heated drying cylinder of a paper machine, wherein thermal insulation formed by a coaxial annular space is provided between the interior space of the hollow shaft and a bearing supporting it.
Configurations of that type are known and are used specifically in conjunction with steam-heated drying cylinders of the drying section of a paper machine. These drying sections serve to dry the paper web that has been dewatered in the press section but is still moist (dry content of 35-50%); the residual water is evaporated in the drying section. The drying section itself consists of steam-heated drying cylinders that can be combined into controllable groups. The drying cylinders are generally heated with steam at a pressure up to about 10 bars. The steam enters sideways through the journals of the drying cylinders. The heat released as the steam condenses is conducted through the relatively thin cylinder walls to the paper web, drying it slowly. The accruing condensate is removed by means of rotating or fixed siphons, generally through the same journal through which the steam is admitted. The maximum temperature in the drying cylinders is about 120.degree.-190.degree. C.
As mentioned before, the steam is admitted through a drying cylinder journal which is fashioned as a hollow shaft or a hollow cylinder. The accruing condensate is then removed by way of the same journal or also a second journal.
Antifriction bearings, specifically self-aligning roller bearings are used for the rotatable mounting of the drying cylinder. These bearings are arranged, for one, on the journals fashioned as a hollow shaft and which, on the other hand, bear on suitable pillow blocks.
These journals, and thus also the bearings themselves, are heated by the steam and/or condensate passing therethrough. In the process, a problem arises since the inner race of the antifriction bearing, press-fitted on the hollow shaft, assumes a temperature higher than the outer race of the antifriction bearing, thereby reducing the play of the antifriction bearing. In the extreme case, this bearing play may be reduced to zero, ultimately giving rise to the risk of destroying the bearing.
In addition to this aspect of risk, the varying operating temperature developing in the bearing affects the lubricating properties and service life of the lubricating oil.
To overcome this problem, various suggestions or approaches of solution have already been made, which in principle are based on providing, between the cylinder bore of the hollow shaft through which the steam or condensate flows and the bearing, a means or measures for thermal insulation.
Known from U.S. Pat. No. 2,413,567 is a drying cylinder wherein the cylinder plates delimiting the interior of the cylinder each include a molded hollow journal and an additional, bolted hollow journal which accommodates the bearing. Provided for steam admission or condensate removal is a stationary device which extends through the molded hollow journal and is sealed relative to the hollow journal by means of packing chambers. The mentioned means for thermal insulation of the bearings consist of a socket of insulating material which is slipped on the molded hollow journal, the outside diameter of which socket is so selected that an air gap is present between the socket and the bolted hollow journal. Due to the two-part journal, this design is relatively expensive.
U.S. Pat. No. 2,817,908 depicts a drying cylinder for paper machines with a double-wall steam inlet pipe wherein the annular space between the two walls acts as an insulation layer. Another thermal insulating space is provided between the outer wall of the steam inlet pipe and the hollow journal itself.
The double-wall steam inlet pipe is, on one end, screwed to the hollow journal while on the other end a movable packing is provided. This packing allows for the fact that the steam inlet pipe expands in the axial direction more than does the hollow journal, and it seals the thermal insulating space between the steam inlet pipe and the hollow journal. In practical use, however, this concept has proved to be problematic as the space cannot be dependably sealed in the long run. Condensate accumulates in the course of time in this intermediate space and the thermal insulating effect is increasingly lost.
Regarding the thermal insulating annular space between the two walls of the steam inlet pipe, it can be durably sealed. But considering that these two walls vary in their axial expansion, an equalization element needs to be provided, for instance a bellows.
Viewed overall, the design according to U.S. Pat. No. 2,817,908 is thus also too complicated and expensive.
U.S. Pat. No. 3,224,110, moreover, shows a drying cylinder where the annular space between a condensate outlet pipe and a hollow journal is filled with thermal insulating material. Here, too, the problem arises that, due to the varying axial expansion of the hollow journal and the condensate outlet pipe a movable packing needs to be provided, the service life of which naturally is limited. Once the effect of the packing is reduced, condensate will eventually seep into the thermal insulating material, so that the insulating effect will be increasingly lost.
Another proposal which has become known for the solution of the initially mentioned problem consists of compensating for the operationally developing temperature difference between the inner and outer races of the antifriction bearing means of a cooling device provided in or on the antifriction bearing, or to at least minimize the temperature difference. This proposed solution has been demonstrated to be insufficient.
The problem underlying the present invention consists of providing a journal of the categorial type which is of simple design and wherein the thermal insulation between the cylinder bore of the hollow shaft serving as a pipe for the heat transfer medium and the inner race of the antifriction bearing is so conceived that no condensate can penetrate and reduce the insulating effect.